It is frequently necessary to obtain a signal to indicate the position of a shaft located in an instrument or measurement device where installing a mechanical or electromechanical contact would hinder the free rotation of the shaft. In the past optical devices have been fabricated to achieve this result, but these devices either have a limited range of angular rotation or are heavy and complex. An optical shaft position sensor is shown in U.S. Pat. No. 3,940,609. This device has a split cylindrical prism affixed to the rotating shaft which deflects light coming from a radial source. The angle at which this light is deflected is dependent upon the position of the shaft and prism. A series of photodiodes are used to indicate the location of the shaft. One problem with this type of device is that travel is limited to about 180.degree. and therefore this device can only be used where shaft position readings are only taken during this limited portion of the shaft's rotation.
Another example of a device which uses a prism affixed to the shaft to determine shaft position optically is shown in U.S. Pat. No. 3,351,767. A light source, the shaft which supports a pair of cylindrical prisms, and a photodiode are all located along a single axis. The prism is designed such that light will only pass through the prism and strike the photodiode when the shaft connected to the prism is oriented so that the light strikes the prism at less than the critical angle. The range of shaft rotation in which light will pass cannot be changed, as it is fixed dependent upon the physical properties of the prism.
An example of another prior art device is shown in U.S. Pat. No. 2,182,717 relating to a control system for a dirigible which senses the position of a compass and adjusts the rudder to maintain the craft on course. A reflector is mounted along the axis of rotation of a compass to deflect a light source mounted above the compass radially to a light sensitive control unit. The control unit may be located at any compass heading, and the shaft is free to turn 360.degree.. The light source is similarly mounted above the compass, resulting in an appearance different than that of a typical compass, and obstructing the user's view of the dial face.
An example of another prior art device used to optically indicate the position of a shaft is an optical position encoder. Encoders are capable of producing a binary signal which will indicate the position of the shaft. The resolution of an optical encoder will vary dependent upon the number of bits or binary codes present in the output signal. The operation of an optical shaft encoder is described in an article by Charles Hudson published in Instruments and Control Systems, May 1978, entitled "A Guide to Optical Shaft Encoders." The primary disadvantage of an optical shaft encoder is that the disk attached to the rotating shaft can substantially increase the inertia of the shaft and degrade performance of the instrument. The size of the optical encoder may also be a problem in some applications.